1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and more particularly, to a zoom lens suitable for use in an image pickup apparatus, such as a video camera, an electronic still camera, a broadcast camera, or a surveillance camera, or an image pickup apparatus such as a camera using a silver-halide film.
2. Description of the Related Art
In recent years, image pickup apparatuses with increased functionality and reduced size are ever more appealing. As an imaging optical system for use in such an image pickup apparatus, there is a demand for a zoom lens having compact size, wide angle of view, high zoom ratio, and high resolving power.
To achieve the desired high zoom ratio and compact size of a zoom lens, the refractive powers (optical powers=reciprocals of focal lengths) of lens units that constitute the zoom lens are generally increased. In such a zoom lens, however, aberration variation due to zooming increases, the front-lens effective diameter increases in a zoom range from a wide angle end to an intermediate zoom position, and this increases the size of the entire zoom lens.
A positive lead type zoom lens is known in which a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a rear group including at least one lens unit are arranged in order from an object side to an image side. The side of a lens where an object to be imaged is placed is referred to as the object side or front side of the lens; and the side of the lens where the image is formed is referred to as the image side or back side of the lens.
U.S. Pat. No. 7,206,137 discloses a zoom lens that includes, in order from the object side to the image side, four lens units respectively having positive, negative, positive, and positive refractive powers.
U.S. Pat. No. 7,286,304 discloses a zoom lens that includes, in order from the object side to the image side, five lens units respectively having positive, negative, positive, positive, and positive refractive powers.
U.S. Patent Application Publication No. 2008/0231969 A1 discloses a zoom lens that includes, in order from the object side to the image side, five lens units respectively having positive, negative, positive, negative, and positive refractive powers.
To obtain a high optical performance in these zoom lenses while ensuring a wide angle of view, a high zoom ratio, and a compact size, it is important to properly set the refractive powers, lens configurations, and moving parameters of each the lens units or components thereof during a zooming operation. In particular, it is important to properly set the refractive powers of the first and second lens units, the lens configuration of the second lens unit, and the moving conditions of the first, second, and third lens units for zooming. It is also important to properly set the position of an aperture stop and the moving condition of the aperture stop during zooming.